In the drilling of wells for the exploration of oil or gas two main types of drill bit are used, roller cone bits and polycrystalline diamond compact (PDC) bits.
The roller cone drill bit is based on three toothed cones being rotated under compression causing the teeth to crush and scoop the rock as they rotate. This method of drilling however is limited by the weight which the cone bearing can tolerate, the life of the cones and the speed of rotation which is typically limited to a maximum of 90 rpm.
The PDC bit achieves the cut by shearing off the top surface of the rock by means of a scraping action. This type of bit depends on high speed rotation but requires a limited weight to prevent stalling.
Other types of drilling methods have been used for example impact drilling and cable drilling where a hammer bit is vertically impounded into the bottom of the hole.
Another type of drilling system is disclosed in U.S. Pat. No. 4,289,210 comprising a plurality of chisel blades which are impacted periodically downwards into the formation being drilled.
All these types of drilling systems may be used for the conventional method of drilling and casing a well.
It is normal practice when drilling into a formation that the initial hole drilled is of a much greater diameter than the final working hole required for the subsequent production of hydrocarbons from the formation.
The parameters that control the size of the first hole are:
(a) the size of the reservoir casing PA1 (b) the casing strings required to achieve safe drilling practices, and PA1 (c) the annular sizes required between the casing and open hole to allow satisfactory cementation. PA1 (i) a vertically extending rotatable member, PA1 (ii) means for drilling a hole fixed to the lower end of said member, and PA1 (iii) a hole enlarging drilling unit located above said drilling means having a number of chisel blades circumferentially angularly offset and retractably mounted on said member, said drilling unit being provided with means for imparting a percussion drilling motion to said chisel blades. PA1 (i) running casing into said enlarged section wherein said casing is of slightly smaller diameter than that present in the previously cased section, PA1 (ii) pumping cement into the annulus formed between the outer surface of said casing and the enlarged section of said formation, PA1 (iii) providing means in said casing to permit a circulation path for said cement between said annulus and an inner portion of said casing, PA1 (iv) securing said casing in position within said hole, PA1 (v) hanging and packing off in the last casing string, and PA1 (vi) latching and sealing the previously cased section to said last casing string.
It can be seen that the starting size of the hole is a straight addition of these factors whereby the largest open hole bit that is run has to fit inside the last casing string.